The mechanisms of HCV eradication following IFN-?/ribavirin therapy in HCV/HIV-1 co-infection or HCV mono-infection remain unclear. The goal of this proposal is to determine the role of innate immunity effectors in therapy response in HCV and HCV/HIV-1 co-infected subjects by investigating Natural Killer (NK) cell and Dendritic Cell (DC) functionality in relation to level of adaptive T cell responses and therapy-induced HCV suppression. We will test the hypothesis that the sustained functional response of innate effector cells (i.e. Natural Killer cell and Dendritic cell function) to IFN-??/ribavirin therapy is a determinant of both innate and level of adaptive (HCV-specific) responses and ultimately early and sustained HCV virologic suppression. As a corollary, we hypothesize that innate phenotypes and cell-mediated responses associated with HCV control would be selectively enriched in subjects with documented SVS (independent of HIV-1 infection) as compared to healthy or HIV-1-infected donors without HCV infection. We will test this hypothesis by two aims on KIR/HLA-C typed HCV-infected and HCV/HIV-co-infected subjects undergoing treatment with peg-IFN- a/ribavirin by: (1) Analyzing a prospective longitudinal cohort of subjects reaching 12 wk EVR with follow-up to SVR, as compared to subjects failing to achieve EVR but remaining on therapy, with data collection for: (a) Levels of NK and DC subset distribution, DC maturation status and overall cellular immune activation by flow cytometry in relation to HCV viral load;(b) Degree of IRF-7 increase within PDC subsets as a reflection of IFN-a/STAT1/IRF-7 feedback loop and retained IFNR-I function, as measured by flow cytometry detection of IFN-a-induced STAT1 phosphorylation in PBMC subsets;(c) NK constitutive and IFN-a-induced lytic function against HLA-null or viral infected targets, including HCV-infected hepatocytes;and (d) Relation between innate functionality and levels of HCV T cell memory responses, as measured by IFN-? ELISPOT and tetramer assay at time of EVR and therapy completion. The second specific aim will analyze a cross-sectional cohort of previous mono-HCV or dual HCV/HIV-infected subjects having achieved HCV suppression and SVR status, as compared to uninfected controls, measuring (a) Flow-cytometric analysis of NK/DC subset frequency;(b) NK cell functional response (cytotoxicity and degranulation, STAT1 phosphorylation, expression of activation markers) to IFN-a, HLA-depleted target cells and HCV-infected hepatocyte;(c) Dendritic cell functional response (cytokine production, IRF-7 expression) to IFN-a and TLR ligands (CpG, Resiquimod). This basic and clinical research study represents a hypothesis-driven collaborative effort by the Wistar Institute, The Jonathan Lax Center for the Treatment of Immune Disorders (Philadelphia FIGHT), The Infectious Disease Division for the University of Pennsylvania, The AIDS clinic of Drexel University, The National Cancer Institute's Laboratory of Genomic Diversity and Laboratory of Experimental Immunology (Frederick, MD), BD Bioscience (San Diego CA), and the Department of Biostatistics of the University of Massachusetts-Amherst. Public Health Relevance Statement PUBLIC HEALTH RELEVANCE: The mechanisms of HCV eradication following IFN-a/ribavirin therapy in HCV/HIV-1 co-infected or HCV mono-infected subjects remain unclear. The goal of this proposal is to determine the role of innate immunity effectors in IFN-a/ribavirin therapy response in HCV and HCV/HIV-1 co- infected subjects by investigating Natural Killer (NK) cell and Dendritic Cell (DC) functionality in relation to level of adaptive T cell responses and therapy-induced HCV viral suppression. As the rate of response to therapy varies from about 60% in mono-infection to <25% in HIV/HCV co-infected individuals, it is imperative that added research be conducted in understanding clearance mechanisms.